Pressure or force transducers are used in a wide variety of applications. Such transducers are often exposed to various forces and torques. Examples are measurements of cutting forces in manufacturing processes, particularly in milling, planing and the like, where a translational movement is performed under force. Depending on the application, a plurality of components of forces and torques in a Cartesian coordinate system with three coordinate axes X, Y, and Z are sensed simultaneously.
Known pressure or force transducers work on the basis of piezoelectric crystal material or strain gauges (DMS). In the case of the piezoelectric crystal material, the sensor element and transducer element are combined in one part while the DMS is only a sensor element and must be connected to a separate transducer element to form a DMS system. There are big differences between the two with respect to function. Thus, the rigidity and, therefore, accuracy, natural frequency, overload capability of the piezoelectric crystal material typically are by an order of magnitude higher than in the case of the DMS system. In addition, the dimensions of a transducer comprising piezoelectric crystal material are smaller by a factor of 30 than a comparable transducer that comprises a DMS system. And finally, the dynamic measuring range of the piezoelectric crystal material is much broader so that a single transducer comprising piezoelectric crystal material covers the dynamic measuring range of multiple transducers comprising DMS systems. For these reasons, the invention relates to a multi-component transducer based on piezoelectric crystal material.
The document CH472668A corresponding to U.S. Pat. No. 3,566,163 discloses a three-component transducer based on piezoelectric crystal material for the simultaneous measurement of three force components. The structural design of this three-component transducer is schematically shown in detail in FIG. 1 of the present specification. A force F indicated by a straight arrow is absorbed by disc-shaped elements 21 made of piezoelectric crystal material. The negative electric polarization charges generated by the force F on element surfaces of the elements 21 are indicated by minus signs − while the positive electric polarization charges generated by the force F on element surfaces of the elements 21 are indicated by plus signs +. The component transducer 20 comprises sensing electrodes 22 for receiving the positive electric polarization charges and it comprises electrodes 23 for receiving the negative electric polarization charges. The sensing electrode 22 provides an output signal that is proportional to the amount of the absorbed force component. The electrode 23 is grounded. Three component transducers 20, each with two elements 21, are stacked on top of each other along the Z coordinate axis forming a group 200 for directly and simultaneously receiving three force components Fx, Fy and Fz. The uppermost transducer 20 measures the force component Fx. The lowermost transducer 20 measures the force component Fy. The intermediate transducer 20 measures the force component Fz.
However, in automation technology and especially in robotics in the case of humanoid robots or as a wrist sensor there is a long-kept need for the acquisition of forces and torques in all three Cartesian coordinates.
To address this, the document CH502590A discloses a six-component transducer for detecting a plurality of components of forces and torques. Four identical three-component transducers based on piezoelectric crystal material are arranged in an X-Y plane spaced apart from each other in the form of a rectangle. The constructional design of each of the four three-component transducers is as known from document CH472668A corresponding to U.S. Pat. No. 3,566,163. The lengths of the sides of the rectangle are many times longer than the diameters of each of the three-component transducers. Thus, each of the three-component transducers simultaneously and directly senses three force components and, subsequently, three further torque components are calculated from the sensed force components and the distances of the three-component transducers from each other by means of vector calculation.
A first object of the present invention is to provide a component transducer based on piezoelectric crystal material for sensing a torque component. Another object of the invention is to provide a multi-component transducer for sensing multiple components of forces and torques using such component transducer wherein said multi-component transducer has as small overall dimensions as possible. Furthermore, the component transducer as well as the multi-component transducer shall be robust in construction and cost-effective in manufacture.